Sea Ray

Never mind.....I figured it out. 

Rattling marbles is never a good sound from a transmission. It usually means a bad or failing bearing and there could be a number of causes for the failure.  I would first change the fluid, keep the port separate from the stbd, pump it out of the dip stick but then open the drain plug and drain the remainder out of the gear case.  Retain the fluid and pass a magnet thru the drained fluid. The magnet will attract the ferrous metals and will indicate what components may be failing.  Then strain the remainder of the fluid thru a rag and examine what you catch. Shiny ferous metals usually mean shafting, bearings; gold colored metal meand brass and is usually from the clutch disks, silver non-metllic metal is aluminum and typically means something is hitting or wearing away the inside of the gear case. 

Hope that gives you a start...........

The noise may be coming from the transmission but is usually caused by a rough idle causing the transmission gears to chuck (fixed gear to ring gear in the gear reduction behind the velvet drive straight transmission in my case). Just found that on mine (inboard 350's with quadrajet carbs) - after a full tune up (which didn't hurt) sprayed carb cleaner around carb to intake and found a leak at the wedge to intake gasket -apparently gasket was incompatible with ethanol, softened the gasket and pieces got pulled into engine leaving a huge gap/vacuum leak. New gasket - problem solved (and cheaply this time!)

Trans pressure is low if idle or plug missing and clutches fail to engage cleanly   Usually not trans but engine issue

I have a similiar noise coming from my star transmission.  The sound only appears when in gear and increases with throttle.  Bruce, I know this was years ago, but did you figure anything out?